The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.
The present invention relates to a lubrication system enabling ceramic bearings to resist mechanical wear at elevated temperatures.
The upper temperature limit of synthetic oils currently used in aircraft turbine engines is approximately 230xc2x0 C. In hi-Mach expendable engine applications, such as in future supersonic cruise missiles, mainshaft support bearing temperatures could be as high as 650xc2x0 C. and consequently, conventional mechanical system technology becomes unacceptable. The bearing temperatures anticipated in these systems are also above the oxidational limit of conventional inorganic solid lubricant formulations such as graphite, molybdenum disulfide, and tungsten disulfide. Though some progress has been made in high-temperature, self-lubricating bearing technology using sodium silicate bonded films containing complex metal chalcogenide solid lubricants on Si3N4 bearing components (see, for example, U.S. Pat. Nos. 5,507,961; 5,089,154; and 5,294,355), bearing life and performance has been unreliable. Also, the coating spray process used for these silicate bonded films can result in undesirable film thickness variations and the bonded films can be susceptible to coating flaking, delamination, and dehydration. Although excellent control of film thickness, adherence, and coating chemical composition could be obtained with other methods, such as plasma spray, ion beam mixing, chemical vapor deposition, and pulsed laser deposition, these require complex and costly film coating apparatus and some are only applicable to simple geometric part shapes. What is desired is a simple method to produce a thin glassy lubricious film.
Accordingly, it is an object of the present invention to provide a novel method to produce high-temperature lubricious glassy films on silicon-based ceramics.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the present invention there is provided a method to produce high-temperature lubricious glassy films on silicon-based ceramics. The method comprises the steps of (a) oxidizing a silicon-based component, (b) coating the oxidized component with an alkali metal compound, and (c) heating the component to a high temperature in a sulfur-rich oxidizing environment.